Device to Device (D2D) communication, i.e., direction communication between user equipments, refers to a mode in which adjacent User Equipments (UEs) can transmit data over a direct link in a short-distance range without the data being forwarded by a central node (i.e., en eNB).
Several typical D2D transmission scenarios in the studies on D2D communication include D2D transmission in coverage-single-cell, in coverage-multi-cell, partial coverage, and out of coverage.
The Long Term Evolution (LTE) D2D technology refers to D2D discovery and communication procedures, controlled by an LTE network, operating in an LTE licensed frequency band. On one hand, the advantages of the D2D technology can be fully utilized, and on the other hand, some problems of the traditional D2D technology, e.g., uncontrollable interference, etc., can be overcome due to the control by the LTE network. The LTE D2D characteristics have been introduced to enable the LTE technology to evolve from the simple radio mobile cellular communication technology toward the universal connectivity technology.
In the LTE D2D system, from the perspectives of a demand for coverage (around 1 kilometer) and a support of Voice over Internet Protocol (VoIP) service (with a small amount of data, and a long period of time for which the service lasts), the LTE D2D system needs to support narrowband transmission, and thus the Frequency Division Multiple Access (FDMA) mode between D2D UEs. Moreover since the position of a transmitting D2D UE (simply Tx UE) in the D2D system is varying, path losses of data transmitted by different Tx UEs up to a receiving D2D UE (simply Rx UE) may significantly differ from each other, so that the “near-far” effect may occur to the Rx UE due to in-band leakage. The so-called “near-far” effect refers to the transmitted data of the Tx UE further from the Rx UE may be affected by in-band leakage from the Tx UE closer to the Rx UE. For example, the Rx UE is receiving the data of the Tx UE 1, and the data of the Tx UE 1 arrives at the Rx UE at the strength of −100 dBmW due to channel fading. The Tx UE 2 nearby the Rx UE also initiates D2D communication concurrently, and the data of the Tx UE 2 arrives at the Rx UE at the strength of −60 dBmW. Even if the transmitted data of the Tx UE 1 and the Tx UE 2 are orthogonal to each other, then the data of the Tx UE 1 will be affected by in-band from such strong in-band leakage from the Tx UE 2 that the Rx UE may not receive the data of the Tx UE 1 correctly.